Osteoarthritis is a degenerative joint disease and is the most common form of arthritis, affecting over 20 million people in America alone, most of which are 45 years old or older. Osteoarthritis causes the cartilage that covers the bone ends to deteriorate, causing pain, inflammation, and disability. Rheumatoid arthritis affects fewer people than osteoarthritis, nonetheless rheumatoid arthritis still affects just over 2 million people in the United States alone. There are also a large number of people who suffer from problems with connective tissue damaged by trauma or injury.
There is a real need for a faster onset of action for the quick relief of pain. Joint inflammation and pain such as that associated with osteoarthritis is the result of increased levels of pro-inflammatory prostaglandins that are derived from arachidonic acid via the enzyme cyclooxygenase. There are two types of this enzyme, COX-1 and COX-2. Non-steroidal anti-inflammatory drugs such as aspirin and ibuprofen reduce the pain and swelling of arthritis by inhibiting the COX-1 form of the enzyme, but have the side effect of causing gastric erosion if used on a regular basis. The newer arthritis drugs such as rofecoxib, and celecoxib, inhibit the COX-2 form of the enzyme, and reduce pain without causing a high incidence of gastric erosion.
In the early 1990s, an inducible isoform of cyclooxygenase (COX) was found. This paved the way for the discovery that COX exists in at least two isoforms; a constitutive “house keeping” form of the enzyme, COX-1, which is responsible for homeostatic functions, and an inducable isoform, COX-2, associated with inflammatory conditions and mitogenic events.
Non-steriodal anti-inflammatory drugs (NSAIDs) such as aspirin, provide pain relief during inflammation by reducing COX-2, but at the expense of also inhibiting the housekeeping or homeostatic functions of COX-1. Part of these homeostatic functions include healing of ulcerations in the stomach, and certain cardiovascular benefits. The NSAIDs are more selective for the COX-1 form of the enzyme, and are thus referred to as COX-1 inhibitors. However, the COX-1 inhibitors also inhibit the COX-2 isoform.
The GI upset and stomach irritation caused by high doses of COX-1 inhibitors is due to their action on prostaglandin production in a manner similar to that of aspirin and aspirin-like anti-inflammatory agents. Numerous studies have shown that the relative incidence of these GI side effects can be correlated to the relative COX-2 specificity of these agents. The higher the specificity for COX-2 over COX-1, the lower the incidence of GI upsets. Accordingly, cyclooxygenase inhibiting agents with increased COX-2 specificity may provide improved anti-inflammatory compositions having less incidences of gastrointestinal distress or side effects.
However, too much selectivity for COX-2 over COX-1 may not be desirable. Certain side-effects may result from COX inhibitors that are extremely selective for COX-2. For example, the cardiovascular benefit of aspirin, a predominantly COX-1 non-steroidal anti-inflammatory drug (NSAID), is thought to be due to its activity as an anti-platelet aggregating drug. COX-2 inhibition does not result in anti-platelet aggregation. Current pharmaceutical COX-2 inhibitors, such as celecoxib or rofecoxib, are highly specific COX-2 inhibitors, and would not be expected to have any COX-1 inhibitory activity. Thus, the cardiac-related side effects that have been noted with the use of some COX-2 specific inhibitors may be related to the lack of any COX-1 inhibition while significantly inhibiting COX-2.
Furthermore, an additional problem associated with highly specific COX-2 inhibitors is the increase in gastric erosion produced by concurrent administration with other non-steroidal anti-inflammatory drugs (NSAIDS). For example, if a patient is taking a highly selective COX-2 inhibitor and also takes aspirin for cardiovascular benefit, the aspirin will cause even worse damage to the gastric mucosa. The reason for this is that some of the prostaglandins that are inhibited by cyclooxygenase inhibitors, such as prostaglandin E-2 (PGE2), are protective of the gastric mucosa, and actually contribute to healing of ulceration. Low dose aspirin produces small erosions in the stomach, and at the site of these ulcerations, the COX-2 enzyme becomes up-regulated. When COX-2 is blocked by selective COX-2 inhibitors, the protection afforded by the beneficial prostaglandins is eliminated. The result is that the ulcerative damage is made even worse. Concomitant administration of selective COX-2 inhibitors with aspirin is therefore contraindicated.
In summary, highly selective single entity COX-2 inhibitors such as rofecoxib and celecoxib, while important new drugs for the treatment of pain associated with osteoarthritis and other maladies, have some serious potential side-effects. These side effects can be divided into two major groups; 1) cardiovascular, and 2) worsening of gastric erosion when taken with aspirin or other NSAIDS. Both of these side effects are related to an unbalanced total inhibition of the COX enzyme, and therefor, virtually complete blocking of prostaglandin production. Because prostaglandins have both positive and negative functions in the body, their total inhibition is a double-edged sword. Furthermore, there is a significant overlap in the patient populations that take both aspirin for cardiovascular benefit, and a selective COX-2 inhibitor for pain. Most of these subjects primarily consist of the elderly population. There is a significant need for anti-inflammatory pain relief without the negative side effects of the NSAIDs or the selective COX-2 inhibitors. Such a composition would provide pain relief while also inhibiting platelet aggregation, and providing protection for the gastric mucosa through some gastroprotective or cytoprotective mechanism. These second generation COX-2 inhibitors would be selective enough to inhibit COX-2 over COX-1, but not so selective that they would result in the additional side effects mentioned above.
In the search for new anti-inflammatory compounds, many potential candidates have come from the plant kingdom. These botanicals are usually extracted and tested in-vitro for COX inhibition using various cell lines and methods. Usually these methods involve screening the compounds for COX-2 and COX-1 inhibition by measuring the inhibition of prostaglandin E-2 for COX-2 inhibition, and TxB2 for COX-1 inhibition. Selectivity can then be determined by calculating the COX-2/COX-1 ratio. But many of these compounds have limited bioavailability in the human or animal gastrointestinal tract. Thus lack of good absorption into the blood stream limits the therapeutic effects of these compounds due to low plasma levels of the active principles.
Part of the poor absorption of botanical COX inhibitors is due in turn to low solubility of these compounds in biological fluids. The pH of the stomach in humans is about 1.2, and in the small intestine, it rises to about pH7.5. Compounds must be somewhat soluble in acidic conditions to provide a fast onset of action. While most compounds are absorbed in the small intestine, they must undergo dissolution and go into solution before they can be absorbed into the blood stream. Ideally, for fast onset of action, a compound should start undergoing dissolution while still in the stomach, and continue dissolution during transit in the small intestine. The compound should therefore be somewhat soluble in the acidic pH of the stomach, as well as the more basic “buffer” conditions that exist in the small intestine.
When screening botanical extracts for COX inhibition in-vitro, a solution of the compound must be made up which is added to the media containing the cells and the other substances. This solution is usually prepared over a range of different concentrations, so that a dose response curve can be calculated. To create a solution of a compound with limited solubility in physiological fluids, a solvent is usually employed. The most commonly used solvent is DMSO, or dimethylsulfoxide, which is somewhat of a universal solvent. But this method produces an artifact that is related to the artificial conditions in which the compound has been put into solution. The fluids in the gastrointestinal tract do not contain solvents such as DMSO or methanol. Many of these botanical compounds are not soluble in water, simulated gastric fluid, or simulated intestinal fluid. One therefore must make a leap of faith when extrapolating these in-vitro results to in-vivo conditions.
It would be desirable to find compounds that exhibit good selective COX-2 inhibition in-vitro, that also have better solubility in physiological fluids. Such compounds would also result in better bioavailability, faster onset of action, and more effective pain relief with less side-effects.
What are needed are compositions and methods that address the problems noted above.